Conventional objective lenses for general endoscopes have a viewing depth in a wide range of roughly 5 to 100 mm on the object side, although they have no focusing function. An endoscope with such an objective lens built in it primarily uses a CCD or other solid-state imaging device to provide images. In recent years, there has been mounting demand for making endoscopic image quality much higher so as to enhance diagnosis accuracy, and CCDs having much more pixels are now under development. The use of CCDs having much more pixels, however, leads to a need of bringing down the F-number of an associated objective lens so as to avoid image quality degradations due to diffraction. In addition, as CCD size grows large under the influence of an increasing number of pixels, it is also necessary to increase the focal length of the objective lens. For one thing and another, the viewing depth now becomes narrow. To make sure the viewing depth on a par with a conventional one, there is thus an increasing need for an objective lens having focusing function.
The primary purpose of such an endoscopic objective lens is that it can be used the same way as a general endoscopic objective lens: it is desired that there be no or little change of viewing with no or little angle-of-view change.
Patent Publication 1 discloses an objective lens having focusing function with limited angle-of-view fluctuations, which comprises two groups, negative and positive, or three groups, negative, positive and positive, wherein focusing is implemented by movement of the second group. Patent Publications 2 and 3 each disclose a two-group construction, positive and positive.
Besides, Patent Publications 4, 5 and 6 disclose an extended endoscopic objective lens capable of focusing on a near distance object point, which comprises three groups, positive, negative and positive, and in which focusing is implemented by movement of the second group. Patent Publication 7 shows an endoscopic objective lens of the type which comprises three groups, negative, positive and negative, and in which focusing is implemented by movement of the second group.